Current sensors using a magneto-resistive element (MR element) have been attracting attention in recent years. Such a current sensor can detect a DC magnetic field and is therefore applicable to various areas including battery control of hybrid vehicles.
Inherently, the MR element is characterized by poor linear characteristics and narrow effective sensitivity range for detecting an external magnetic field. When adopting the MR element for magnetic sensors, therefore, it has been required to shift the operating point to an area having excellent linear characteristics, for example, by applying a bias magnetic field to the MR element, as disclosed in Japanese Unexamined Patent Application Publication Nos. 10-319103 and 6-294853. However, this technology limits the output of the MR element or the like, causing a problem of reducing the freedom of design.
In the field of the magnetic sensor, therefore, a magnetic balance system has been widely used as means for solving the problem. The magnetic balance system is a method in which a coil is disposed around a MR element, a current is applied by feedback control to let the coil generate a measuring magnetic field counterbalancing an external magnetic field, and a strength of the external magnetic field to be measured is obtained from a value of the current (see Japanese Unexamined Patent Application Publication No. 11-64474). In principle, excellent linear characteristics and wide effective sensitivity range can be secured by the magnetic balance system, so that this system is suitable for a current sensor for measuring a high-voltage direct current, such as a battery monitoring sensor of a hybrid vehicle.
Thus, the MR element is expected to be applied to the field of the magnetic sensor. Then, such expectations are running high since a high-performance spin valve-type MR element has been developed along with development of a magnetic head of a hard disk. Examples of the spin valve-type MR elements include a CIP (current in-plane) type GMR element using a giant magneto-resistive effect in which a current flows in a film plane, a CPP (current perpendicular to plane) type GMR element in which a current is passed perpendicular to a film plane, and a TMR element in which a current is passed using a tunneling effect caused by applying a voltage perpendicular to a film plane.
Among them, the TMR element is particularly excellent in output characteristics, so that it is expected that the S/N ratio of the system can be improved and the circuit configuration can be simplified by applying it to the field of the magnetic sensor.
However, the spin valve-type MR element not only has poor linear characteristics as described above but also has hysteresis characteristics with a high coercive force Hc. In the case where the above magnetic balance system is applied, accordingly, if a control current overshoots, there is a problem that the external magnetic field cannot be measured correctly. More specifically, once the MR element has reached magnetization saturation by overshoot, then, under the influence of hysteresis, the magnetic field will be detected at an unintended route portion of the hysteresis curve, thereby causing a measurement error of the magnetic field strength approximately corresponding to the coercive force Hc.